1. Field of the Invention
The present invention relates to a communication connecting device and a data output control method advantageously applicable to, e.g., a gateway that connects a G3 (Group 3) facsimile apparatus to an IP (Internet Protocol) network. More particularly, the present invention relates to a communication connecting device and a data output control method feasible for a real-time facsimile apparatus or terminal for sending UDP (User Datagram Protocol)/IP packets while monitoring the data of the packets.
Generally, for real-time facsimile communication over an IP network, a system configuration proposed by ITU-T (International Telecommunication Union-Telecommunication standardization sector) Recommendation T.38 is used. In this system configuration, an Internet facsimile apparatus or gateway is connected to the IP network at each of the transmitter and receiver sides. The Internet facsimile apparatuses each are connected to a particular G3 facsimile apparatus by a PSTN (Public Switched Telephone Network).
The Internet facsimile apparatus at the transmitter side temporarily stores data input from the sending G3 facsimile apparatus. The facsimile apparatus then packetizes the data by referencing packet size information fed thereto. The resulting packets are referred to as IFP (Internet Facsimile Protocol) packets.
UDP is applied to communication between the Internet facsimile apparatuses. For example, a UDP header is added to the head of a UDP payload storing data. Even when UDP packet data is lost, UDP does not execute processing for reconstructing the packet data.
Specifically, the Internet facsimile apparatus at the transmitter side temporarily stores the IFP packets to be sent. The Internet facsimile apparatus then writes every new IFP packet in the primary field of its storage area to thereby generate a UDPTL (facsimile UDP Transport Layer protocol) payload and send the UDPTL payload to the IP network. In addition, to prepare for the loss of the UDP packet data, the Internet facsimile apparatus writes the UDP packets sent in the past in the secondary field of the storage area to thereby form a UDPTL payload although the past UDP packets are redundant. Sequence numbers unique to the primary field are attached to the primary field. A UDPTL header is added to the head of the UDPTL payload to complete a UDPTL packet. The UDP payload is constituted by such a UDPTL packet. A UDP packet is made up of a UDP header and a UDP payload.
After an IP header has been added to the UDP packet, the UDP packet is sent to the IP network via a LAN (Local Area Network).
The Internet facsimile apparatus at the receiver side decomposes the received IP packets to the level of UDPTL packets. The Internet facsimile apparatus then classifies, among the IFP packets of the decomposed UDPTL packets, IFP packets to be used and then depacketizes the classified IFP packets by decoding them, thereby recovering the original data. The Internet facsimile apparatus temporarily stores the recovered data and then sends them to the receiving G3 facsimile apparatus.
The Internet facsimile apparatus sends a sequence of IFP packets in the form of consecutive UDPTL packets in accordance with the Recommendation T.38 protocol. At this instant, the IFP packets are repeatedly sent the same number of times up to, among a continuous sequence of data, data produced by subtracting the preselected number of storing regions of the secondary field of a UDPTL packet from the last IFP packet. However, consecutive data following the above data are sent a smaller number of times. Consequently, not all of the IFP packets are sent with constant reliability.
More specifically, in the event of continuous transmission based on the UDPTL protocol, IFP packets fed are again sent in the secondary field of the next UDPTL packet. However, if the next IFP packet is absent, then the UDPTL packet is not again sent. This brings about a difference in the frequency of transmission.